Patent Literature 1 discloses a technique of driving a stepping motor by PWM control. According to this technique, control based on a “charge mode”, a “fast decay mode”, and a “slow decay mode” is repeated for each PWM period. Here, the charge mode is an operation mode in which the current supplied to the stator winding is increased, and the fast decay mode is an operation mode in which the current is decreased at a high speed, and the slow decay mode is an operation mode in which the current is decreased at a low speed. In the following description, the fast decay mode and the slow decay mode are collectively referred to simply as a “decay mode” in some cases.
These operation modes are switched one another based on a comparison between a measured current value and a target value of the current supplied to the stepping motor (for example, a waveform approximating a sine wave by a stepwise wave). That is, if the measured current value is equal to or less than the target value, the charge mode is selected, and when the measured current value exceeds the target value, the decay mode may be selected. However, in any of the operation modes, it is difficult to predict the waveform of the measured current value in advance. First, the current waveform in the charge mode varies depending on a drive voltage of the motor, a rotational speed of the motor, a load torque condition of the motor, a temperature environment, and the like.
Also, since the inductance of the stator winding varies depending on the positional relationship between a rotor and a stator, the decay speed of the current in the decay mode also varies according to this positional relationship. When the measured current value deviates from the predicted value, for example, a situation where the measured current value drops greatly occurs, the ripple of the current waveform becomes large in order to compensate for the drop. As a result, torque loss, oscillation, noise, or the like of the motor occurs, and frequent switching of the coil current direction between the charge mode and the fast decay mode may cause electromagnetic noise.
In order to cope with such a problem, in the technique of Patent Literature 1, two comparators are provided to compare the measured current value with two reference values, and the operation mode is switched based on the comparison result and the time. As another method, a method to suppress the ripple by shortening the PWM period is also conceivable.